Method and apparatus for compensating for phase noise of symbols spread with a long spreading code

ABSTRACT

A method and apparatus for compensating for phase noise of symbols spread with a long spreading code are disclosed. To compensate for the phase noise, a phase error estimate is generated from despread symbols with a short spreading code. A phase correcting phasor is applied to chip rate data before despreading the data with a long spreading code. A signal-to-interference ratio (SIR) on a common pilot channel (CPICH) may be calculated by spreading the data with a parent spreading code in an orthogonal variable spreading factor (OVSF) code tree and by combining symbols. Alternatively, a magnitude of the symbols may be used in estimating the SIR. The SIR of a channel using a short spreading code and an SIR of a channel using a long spreading code are measured. The SIR of the channel with the long spreading code may be compensated in accordance with a difference between degradation of the SIRs.

RELATED APPLICATIONS

This application is a continuation of U.S. Pat. No. 7,561,615 that issued on Jul. 14, 2009, which in turn claims priority to U.S. Provisional Applications 60/662,976; 60/663,874; and 60/665,122 filed Mar. 18, 2005; Mar. 21, 2005; and Mar. 25, 2005 respectively. All of these patents and applications are incorporated by reference as if fully set forth.

FIELD OF INVENTION

The present invention is related to a code division multiple access (CDMA) wireless communication system. More particularly, the present invention is related to a method and apparatus for compensating for phase noise of symbols having a long spreading code.

BACKGROUND

In a CDMA system employing different lengths of spreading codes, receiver imperfections, such as phase noise, may degrade transmissions more when using a longer spreading code than using a shorter spreading code if the nature of the imperfections is time-varying, such as phase noise on the scale of the spreading code length. For example, in a universal mobile telecommunication system (UMTS) frequency division duplex (FDD) system, the spreading codes may vary from 4 to 512 chips.

In a third generation (3G) high speed downlink packet access (HSDPA) system, adaptive coding and modulation (AMC) is based on a channel quality indication (CQI) estimated by a wireless transmit/receive unit (WTRU). The CQI is expected to reflect the channel quality of the high speed physical downlink shared channel (HS-PDSCH), which uses a spreading factor (SF) of 16. However, the CQI is generated based on a signal-to-interference ratio (SIR) measured on a common pilot channel (CPICH), which has an SF of 256. In an ideal radio environment, this does not present a problem because different processing gains due to different SFs are easily factored into the CQI generation. However, phase noise can impact the SIR measurements made on signals of different SFs by different amounts. Therefore, the CQI measurement based on the CPICH may not reflect the channel quality seen by the HS-PDSCH.

SUMMARY

The present invention is related to a method and apparatus for compensating for phase noise of symbols having a long spreading code. In order to compensate the phase noise, a phase error estimate is generated from the despread symbols with a short spreading code. The phase correcting phasor generated from the phase error estimate is applied to the chip rate data and then the phase corrected data with long spreading codes is despread. A SIR on a CPICH may be calculated by despreading the chip rate data with a spreading code which is a parent code of a spreading code of a CPICH in an orthogonal variable spreading factor (OVSF) code tree and by combining symbols. Alternatively, a magnitude of the CPICH symbols may be used in estimating the SIR. The SIR of a channel using a short spreading code and an SIR of a channel using a long spreading code are measured. The SIR of the channel using the long spreading code may be compensated in accordance with a difference between degradation of the SIR on the channels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a receiver for compensating phase noise for data spread with a long spreading code in accordance with the present invention.

FIG. 2 is a flow diagram of a process for compensating phase noise for data spread with a long spreading code in accordance with the present invention.

FIG. 3 is a block diagram of an apparatus for compensating phase noise in SIR estimation for long SF symbols in accordance with one embodiment of the present invention.

FIG. 4 is a block diagram of an apparatus for compensating phase noise in SIR estimation for long SF symbols in accordance with another embodiment of the present invention.

FIG. 5 is a block diagram of an apparatus for compensating phase noise in SIR estimation for long SF symbols in accordance with yet another embodiment of the present invention.

FIG. 6 shows degradation of CPICH SIR and HS-PDSCH SIR in the presence of radio impairments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The features of the present invention may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components.

The present invention is applicable to any wireless communication system including, but not limited to, a third generation partnership project (3GPP) system. The present invention will be explained with reference to a CPICH and an HS-PDSCH hereinafter. However, it should be noted that the reference to the CPICH and the HS-PDSCH is for illustration of the present invention and the present invention may be applied to any other channel using any SF.

FIG. 1 is a block diagram of a receiver 100 for compensating for phase noise associated with data spread with a long spreading code in accordance with the present invention. The receiver 100 includes a first despreader 102, a constellation correction unit 104, a buffer 106, a phasor generator 108, a multiplier 110, a second despreader 112, an SIR estimator 114 and a CQI generator 116. A first channel transmits data spread with a short spreading code and, simultaneously, a second channel transmits data spread with a long spreading code. The transmitted data is received and processed to generate a chip rate data 122. The chip rate data 122 is fed to the buffer 106 and the first despreader 102. The buffer 106 temporarily stores the chip rate data 122. The first despreader 102 despreads the chip rate data 122 with a short spreading code 123 to generate symbols 124. The first despreader 102 may be an HS-PDSCH despreader for despreading HS-PDSCH transmissions using an SF of 16.

Symbols 124 generated by the first despreader 102 are fed to the constellation correction unit 104. The constellation correction unit 104 corrects gain and phase errors in the constellation prior to mapping symbols 124 into phase corrected symbols 125. The details of the constellation correction unit 104 and the process for correcting the gain and phase errors are described in a U.S. patent application Ser. No. 10/980,692 filed Nov. 3, 2004 entitled “WIRELESS COMMUNICATION METHOD AND APPARATUS FOR PERFORMING POST-DETECTION CONSTELLATION CORRECTION,” which is incorporated by reference as if fully set forth.

The phase error estimate 126 of each symbol is calculated from the constellation correction unit 104. The phase error estimates 126 are preferably collected over time to generate a smoothed phase error estimate, which may be generated by filtering the phase error estimates or by performing a polynomial fit to the data. The phase error estimate 126 is fed to the phasor generator 108. The phasor generator 108 generates a unit magnitude phasor 130 to correct the phase error in the chip rate data 122. The unit magnitude phasor 130 is multiplied with buffered chip rate data 132 in the buffer 106 via the multiplier 110 to generate phase corrected chip rate data 134.

The phase corrected chip rate data 134 is sent to the second despreader 112. The second despreader 112 despreads the phase corrected chip rate data 134 with a long spreading code 135. The long spreading code 135 may be any length of spreading codes. For example, the second despreader 112 despreads the phase corrected chip rate data 134 with spreading codes for a CPICH, a dedicated channel (DCH), a high speed-shared control channel (HS-SCCH) (or any other channels) and outputs CPICH symbols 136 and DCH and/or HS-SCCH symbols 137.

The CPICH symbols 136 are fed to the SIR estimator 114 for calculating an SIR estimate 138 on the CPICH. The SIR estimate on the CPICH is fed to the CQI generator 116 for generating a CQI 140.

The phase corrected chip rate data 134 may be re-despread for the short spreading codes iteratively with multiple phase error corrections. Additional short spreading code despreaders, constellation correction units and phasor generators may be added so that the output of the additional despreader may again be used to do more constellation correction, phase error estimates and correction.

FIG. 2 is a flow diagram of a process 200 for compensating phase noise for data spread with a long spreading code in accordance with the present invention. A chip rate data is generated by sampling and descrambling received signals (step 202). The chip rate data is stored in a buffer temporarily (step 204). The chip rate data is despread with a short spreading code (step 206). A phase error estimate is generated from symbols obtained by despreading the chip rate data with the short spreading code (step 208). A phase correcting phasor is then generated from the phase error estimate (step 210). The phase correcting phasor is applied to the chip rate data stored in the buffer before despreading the chip rate data with the long spreading code (step 212).

FIG. 3 is a block diagram of an apparatus 300 for compensating for phase noise in SIR estimation for long SF symbols in accordance with one embodiment of the present invention. The apparatus 300 includes a despreader 302, a symbol combiner 304 and an SIR estimator 306. In order to alleviate the impact of the phase noise on symbols spread with a long spreading code, a shorter spreading code is used to despread the symbols and soft symbols output from the despreader 302 is combined to obtain the long SF symbols. The despreader 302 despreads a post-equalizer chip rate data 312 with a short spreading code and the symbol combiner 304 combines the symbols 314 output from the despreader 302 in accordance with timing for CPICH symbol boundary, which will be explained in detail hereinafter. The combined soft symbols 316 may be sent to the SIR estimator 306 to calculate a CPICH SIR 318.

For example, for the case of 3G FDD, a CQI is generated based on a CPICH SIR estimate. The CPICH SF is 256 and the chip rate is 3.84 Mchips/s. If the SF of 64 is used for de-spreading, then four (4) consecutive soft symbols are combined to estimate the CPICH symbol. A timing signal 320 is provided to the soft symbol combiner 304 such that the soft symbols 314 to be combined are aligned to the CPICH symbol boundary.

For the above example, (a CPICH spreading with an SF=256 code and despreading with an SF=64 code), despreading with a short spreading code and symbol combining are explained hereinafter. {right arrow over (s)}=[s₁ s₂ s₃ s₄]^(T) represents a column vector of soft symbols at the de-spreader output. {right arrow over (d)}=[d₁ d₂ d₃ d₄]^(T) represents a column vector of symbols transmitted for each of the 4 codes with SF=256 derived from the common SF=64 parent code in an OVSF code tree. The common SF=64 parent code corresponds to the OVSF tree branch that the CPICH belongs to. H₄ represents a 4^(th) order Hadamard matrix.

In the absence of noise, the soft symbols output from the despreader 302 are written as follows:

{right arrow over (s)}=H ₄ ·{right arrow over (d)}.  Equation (1)

The transmitted symbols are estimated from the despread soft symbols as follows:

{right arrow over ({circumflex over (d)}=H₄ ⁻¹ ·{right arrow over (s)}.  Equation (2)

Using the well known property of the Hadamard matrix: H_(N)·H_(N) ^(T)=N·I_(N), Equation (2) can be rewritten as follows:

$\begin{matrix} {\overset{\hat{\rightarrow}}{d} = {\frac{1}{4} \cdot h_{4}^{T} \cdot {\overset{\rightarrow}{s}.}}} & {{Equation}\mspace{14mu} (3)} \end{matrix}$

For applications where only the CPICH symbols are of interest, there is no need to perform matrix multiplication. The matrix multiplication can be replaced with a vector dot-product operation.

FIG. 4 is a block diagram of an apparatus 400 for compensating for phase noise in SIR estimation for long SF symbols in accordance with another embodiment of the present invention. The apparatus 400 includes a deapreader 402, a magnitude calculator 404 and an SIR estimator 406. Post-equalizer chip rate data is despread with a long spreading code and an SIR estimate is calculated by using the symbol magnitude instead of the complex symbol. The post-equalizer chip rate data 412 is despread by the despreader 402 using the same spreading code used in transmission, (i.e., a long spreading code). The symbols 414 are then fed to the magnitude calculator 404 for calculating magnitude of the symbols. The SIR estimator 406 uses the magnitude values 416 for calculating a CPICH SIR 418.

FIG. 5 is a block diagram of an apparatus 500 for compensating phase noise in SIR estimation on a channel using a long spreading code in accordance with yet another embodiment of the present invention. The apparatus 500 includes an SIR estimator 502, a mapping unit 504 and a CQI generator 506. The SIR estimator 502 estimates an SIR on both a channel using a short spreading code and a channel using a long spreading code. The SIR measured on a channel using the long spreading code is then mapped to a compensated SIR by the mapping unit 504. For example, a measured SIR on a CPICH (which uses a long spreading code) is compensated by the channel quality seen by a HS-PDSCH (which uses a short spreading code). The compensated SIR is then mapped to a CQI by the CQI generator 506.

The CPICH SIR mapping is performed in accordance with performance difference anticipated for different SFs in the presence of phase noise or other radio impairments. Different types of radio impairments can be isolated and simulated to quantify the difference in performance of different spreading codes with different SFs. For example, for any given set of radio impairments, simulations can be run for a range of SIR values to measure the degradation of both the CPICH SIR and the HS-PDSCH SIR with respect to the ideal values. The difference between the degradation of the CPICH SIR and the degradation of the HS-PDSCH SIR can then be used to bias the CPICH SIR or the CQI. In this way, the CQI correctly reflects the channel quality experienced by the HS-PDSCH.

Once the difference in performance is quantified, a framework for the compensation can be constructed. An example of how the various radio impairments have different impact on the long SF symbols (e.g., CPICH symbols) versus the short SF symbols (e.g., HS-PDSCH symbols) for a 3G FDD system, is shown in FIG. 6.

The mapping by the mapping unit 504 may be implemented as an equation evaluation or a as a look-up table (LUT). The compensation may be implemented prior to mapping to a CQI, or alternatively, may be applied directly to the CQI generated by an uncompensated CPICH SIR.

Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention. 

1. A receiver that receives data with a short spreading code transmitted over a first channel and data with a long spreading code transmitted over a second channel, comprising: a despreader that receives transmitted data that has been processed as chip rate data, and that despreads chip rate data generated from received signals with a short spreading code to generate symbols; a constellation correction unit that generates a phase error estimate based on the symbols; a phasor generator that generates a phase correcting phasor based on the phase error estimate; a multiplier that multiplies the phase correcting phasor with the chip data rate to generate phase corrected chip rate data; and a second despreader that receives the phase corrected chip rate data and despreads the corrected chip rate data with a long spreading code.
 2. The receiver of claim 1 wherein the first channel is a high speed physical downlink shared channel (HS-PDSCH) and the second channel is a common pilot channel (CPICH).
 3. The receiver of claim 2 further comprising a signal-to-interference ratio (SIR) estimator for estimating an SIR from the second channel symbols on the CPICH channel.
 4. The receiver of claim 3, further comprising a channel quality indicator (CQI) mapping unit for generating a CQI from the SIR on the CPICH.
 5. The receiver of claim 2, wherein the despreader is an HS-PDSCH despreader that uses a spreading factor of
 16. 6. The receiver of claim 1, wherein the constellation correction unit corrects gain and phase errors.
 7. The receiver of claim 1, wherein the phase error estimate is an average of multiple phase error estimates generated by the constellation correction unit.
 8. The receiver of claim 1, further comprising a magnitude calculator that calculates a magnitude of the symbols.
 9. The receiver of claim 1, further comprising an SIR estimator that estimates a signal to interference ratio (SIR) SIR using the magnitude of the symbols.
 10. A receiver that receives data with a short spreading code transmitted over a first channel and data with a long spreading code transmitted over a second channel, comprising: a first despreader that receives transmitted data that has been processed as chip rate data, and that despreads chip rate data generated from received signals with a short spreading code to generate symbols; a buffer that receives and stores the chip rate data; a constellation correction unit that generates phase corrected symbols based on the generated symbols; a phasor generator that generates a unit magnitude phasor from phase error estimates received from the constellation correction unit; a multiplier that multiplies the unit magnitude phasor and chip rate data to generate phase-corrected chip rate data; a second despreader that receives the phase-corrected chip rate data and despreads the corrected chip rate data with a long spreading code.
 11. The receiver of claim 10 wherein the first channel is a high speed physical downlink shared channel (HS-PDSCH) and the second channel is a common pilot channel (CPICH).
 12. The receiver of claim 11 wherein the second despreader generates dedicated channel (DCH) high speed control channel (HS-PDSCH) symbols.
 13. The receiver of claim 10 wherein the second despreader generates common pilot channel (CPICH) symbols.
 14. The receiver of claim 13 further comprising a signal to interference ratio (SIR) estimator generates an SIR estimate from the CPICH symbols.
 15. The receiver of claim 14 further comprising a channel quality indicator (CQI) generator that generates a channel quality indication from the SIR estimate. 